Schottky diode

Commercially available Schottky diodes in different housings

A Schottky diode , also known as a hot carrier diode , is a special diode in electronics that does not have a pn junction ( semiconductor- semiconductor junction) but a (blocking) metal-semiconductor junction . This interface between metal and semiconductor is called a Schottky contact or, based on the potential barrier that occurs, a Schottky barrier . Like the pn junction, the Schottky diode is also a rectifier . When Schottky diodes the material composition is (z. B. doping of the semiconductor and the work function of the metal) is selected so that in the interface in the semiconductor, a depletion region is formed. The non-linear Schottky contact thus differs from metal-semiconductor junctions under other conditions, such as the ohmic contact , which shows the behavior of a partially linear ohmic resistance .

history

The Schottky diode is named after the German physicist Walter Schottky , who developed the model of the metal-semiconductor contact in 1938. The rectifying properties were first observed by Ferdinand Braun in 1874 . Initially, the metal-semiconductor junctions consisted of point-like contacts that were made with a pointed metal wire on a semiconductor surface ( tip diode ). In the middle of the 20th century, they were used primarily in the detector receivers customary at the time . The first Schottky diodes, then known as crystal detectors , turned out to be very unreliable. The point-like contact was therefore replaced by a thin metal film, which is still the case with today's commercial Schottky diodes.

Schottky diodes in electronics

Embodiments of Schottky diodes 1. MESH diode, 2. Passivated diode, 3. Offset junction diode, 4. Hybrid diode

Circuit symbols (not standardized)

Circuit symbol according to DIN EN 60617

As “fast” diodes, Schottky diodes are suitable for high-frequency applications up to the microwave range, which is mainly due to their small saturation capacities. That is why they are also often used as protective diodes to reduce the voltage of induction voltages ( freewheeling diodes ) or as rectifier diodes in switched-mode power supplies , where they enable switching frequencies of over 1 MHz. They are also well suited as demodulators for detector circuits .

As a semiconductor material for voltages up to 250 V usually silicon , for blocking voltages 300-1700 V and gallium arsenide (GaAs), silicon carbide (SiC), or SiGe is used.

Silicon Schottky diodes

Silicon Schottky diodes have a lower threshold voltage of approx. 0.4 V. With a very low operating current, the voltage drop can even drop below 0.1 V. This is significantly less than with a silicon pn junction with approx. 0.7 V. They can therefore be connected in parallel to the collector-base path of a silicon bipolar transistor in order to prevent the transistor from saturating and thus switching much faster of the transistor in the blocking state. This was especially the case before the spread of powerful MOSFETs in fast switches such as B. used in switching power supplies, but also for the implementation of faster TTL logic circuits ( digital technology ) z. B. in rows 74 (A) S and 74 (A) LS.

The inherent disadvantage of silicon Schottky diodes is the higher leakage currents compared to the silicon-based pn diode, as well as the conduction losses, which increase rapidly when designed for a higher reverse voltage.

Silicon carbide Schottky diodes

Schottky diodes based on silicon carbide (SiC) have a threshold voltage of approx. 0.8 V, but offer a number of advantages in power electronics over conventional silicon diodes. SiC Schottky diodes are available up to reverse voltages of 1.7 kV, which means that they are used in particular in the field of power electronics such as switched-mode power supplies and converters . Since they have almost no forward and above all backward recovery behavior, they come very close to the ideal diode. When used as a commutation partner for Insulated Gate Bipolar Transistors (IGBT), a considerable reduction in switching losses in the diode itself, but also in the transistor, is possible, since it does not need to take over any reverse recovery current when it is switched on again. The permissible junction temperatures in the case of corresponding housings are up to 200 ° C significantly higher than with silicon Schottky diodes, which simplifies the cooling of SiC diodes.

function

Band diagrams (n-type) metal-semiconductor transition. Left both individual materials and right equilibrium situation after contact

The function of a Schottky diode with n-doped semiconductor material (the usual design) is now dealt with using the ribbon model , in which the potential energy of the electrons is plotted as a function of the location. In a simplified approach, it is often assumed that a metal (left in the picture) and a semiconductor (right of it) are joined together without the electronic structure changing due to the metal-semiconductor bond in the solid state of metal and semiconductor. Assuming that the work function of the metal greater than the electron affinity of the semiconductor, is - what is fulfilled for most metal-semiconductor combinations, which are used for Schottky diode - that is generated at the interface between the edge Fermi of Metal and the lower edge of the conduction band of the semiconductor a potential level of height . ${\displaystyle q\cdot \varphi _{m}}$ ${\displaystyle q\cdot \chi }$ ${\displaystyle W_{\text{F}}}$ ${\displaystyle \varphi _{\text{B}}=\varphi _{\text{m}}-\chi }$

In reality, however, the surfaces of metal and semiconductors are greatly altered by the bond and the actual height of the potential level or Schottky barrier is primarily determined by the metal-semiconductor bond, but also by process parameters such as the cleaning of the surface and hardly by the Work function of the metal dependent. For n-Si the Schottky barrier is mostly between 0.5 and 0.9 eV. ${\displaystyle \varphi _{\text{B}}}$

The Fermi energy of the undisturbed (n-doped) semiconductor is (except in the case of degenerate semiconductors) just below the conduction band. In the case of metal / semiconductor contact, there is a charge equalization, the Fermi energies of the two partners are equal, there is only one common Fermi energy W _F (x, t) = const in thermodynamic equilibrium. Due to the different work functions of the two partners, charge influence occurs on the two surfaces. Electrons collect on the metal surface and flow away from the semiconductor surface and thus generate positive impurities in the semiconductor. A potential wall is created and the strips of the semiconductor “bend”. About the band bending the electrons can leave the semiconductor, it creates a so-called depletion zone (Engl. Depletion zone ) in which the potential energy of the electrons in the conduction band ( majority carriers ) is high. ${\displaystyle W_{\text{F}}}$

As stated, the electrons in the semiconductor have a higher energy state than the electrons in the metal. They are therefore also called “hot charge carriers”. This is where the term “hot carrier diode” comes from (German: hot charge carrier diode ).

If a positive voltage is now applied (negative pole on the n-type semiconductor), electrons from the semiconductor material are forced into the depletion zone and the potential barrier becomes smaller. Electrons can then flow from the semiconductor into the metal (“ forward bias” ). If, on the other hand, a negative voltage is applied (which is not too high), the electrons are drawn even more strongly towards the metal, the thickness of the depletion zone increases ( reverse bias ). There is only a very small current because a few electrons of the metal can overcome the barrier by thermal excitation or “tunnel” through the barrier (quantum mechanical tunnel effect ). However, if the voltage in the reverse direction is too high, a breakdown occurs.

In the Schottky junction, the minority charge carriers do not contribute to the charge transport. Since the electrons (majority charge carriers) follow the electric field very quickly, the Schottky diode is significantly faster than normal semiconductor diodes , which are based on a pn junction , especially during the transition from forward to reverse operation . With Schottky diodes made of silicon, switching frequencies of more than 10  GHz are possible , and of GaAs or InP even of more than 100 GHz.

Ohmic contact

→ Main article : ohmic contact

Not every metal-semiconductor contact has a rectifying effect. Since the thickness of the depletion zone is inversely proportional to the root of the charge carrier density of the donor, if the semiconductor is heavily doped, the barrier becomes so narrow that it can be neglected and the contact behaves like a small ohmic resistance . Furthermore, the Schottky junction can become an ohmic contact through alloying (formation of silicides at the boundary). Ohmic contacts are required in order to be able to contact semiconductor chips with metallic connecting wires at all.

swell

• Ludwig Bergmann, Clemens Schaefer, Wilhelm Raith: Textbook of Experimental Physics . Solid. tape 6 . deGruyter, 1992, ISBN 3-11-012605-2 .

See also

• List of components (electrics / electronics)
• Schottky effect

literature

• Harald Ibach, Hans Lüth: Solid State Physics . 7th edition. Springer, 2009, ISBN 978-3-540-85794-5 .
• Stefan Goßner: Basics of electronics . 9th edition. Shaker, 2016, ISBN 978-3-8265-8825-9 .

Web links

Commons : Schottky diodes  - collection of images, videos and audio files

Individual evidence

1. ^ A. Lindemann, St. Knigge: Electrical Behavior of a New Gallium Arsenide Power Schottky Diode. (PDF; 213 kB) IXYS , July 26, 1999, accessed on December 13, 2013 (English). 
2. ↑ SiC Schottky diodes from Infineon
3. ↑ ^{a b} Power Electronics Technology  ( page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. Schottky Diodes: the Old Ones Are Good, the New Ones Are Better